SERVICEMAN’S LOG
Lightning strikes thrice
A popular superstition when I was a youngster
was that various calamities – natural or manmade – always happened in threes. Well, that
was according to the adults who knew all about
these things. I never did believe it but a recent
experience does have me wondering a little.
The story is really about video
recorders; and I use the plural term
deliberately because it involves no
less that four machines, all of the
same brand – but different models
– which turned up on the bench in
quick succession. And three of them
had suffered lightning strikes. See
what I mean!
The first one was a Panasonic NVJ1A, a run-of-the-mill model now
about four years old and no longer in
production. It is owned by one of my
regular customers who, incidentally, is
someth­ing of a computer buff, a point
of some interest as it tran­spired.
He came into the workshop one
morning, pushed the machine across
the counter with the cover removed
and said, “Will you put a fuse in it.
It’s stopped and the fuse is blown”,
pointing to the offending component.
Well, he wasn’t wrong; one glance
at the blackened glass was enough
to confirm that. But it was obvious
from the way he spoke that he firmly
believed that the fuse was the only
thing wrong. And when I gently suggested that this was an effect, rather
than a cause, he was quite reluctant
to accept the idea.
There is only one answer in such
cases. I fished out an appropriate replacement, fitted it and applied power.
Splat! – one fuse destroyed. After
that, he didn’t argue. But it was only
then that he told me about the storm
and the fact that his power main had
apparently been struck.
And so he agreed to leave the machine with me. But I warned him that,
if he was lucky, the damage would be
confined to the power supply. If he
wasn’t, it could be a lot more serious.
I didn’t attack it immediately. There
were other jobs ahead of it, the owner
had indicated that he was not in a
hurry, and I was not at all familiar with
this power supply. I had a manual but
had not had occasion to service this
section before. All I knew was that
it appeared to be fairly conventional
switchmode supply.
Strike two
In any case, before I could get it on
the bench, what should turn up but
another Panasonic recorder; an NVFS90A. This is a very much up-market
Super VHS model, with all the bells
and whistles one can imagine – and a
price tag to match. But for all that, its
power supply is almost identical with
the NV-J1A.
And it had also suffered a lightning
strike; not from the same storm but
40 Silicon Chip
Fig.1: the power supply circuit for the Panasonic NV-FS90A video recorder. The components to be checked out
included the mains fuse (F1101), the bridge rectifier (D1102), the starter circuit across the output of the bridge
rectifier, the auto vol­tage selector IC (IC1101), and safety resistor R1125.
from one a couple of days later. In this
case, however, there were no obvious
symptoms and the fuse was still intact.
Its owner was anxious to get it fixed
as quickly as possible.
In all these circumstances, I decided
to let the FS90A jump the queue. After
all, I had to familiarise myself with
the power supply on one machine or
the other, so it might as well be on the
more urgent one.
The power supply is a self-contained unit which is housed in a metal
box. It sits in the rear lefthand corner
and is easily removed by undoing a
few screws. The box itself measures
about 150 x 50 x 65mm and the bottom cover can be sprung off quite
easily, although the top can present
problems.
Not all models have the same top.
Some, including these two models,
cover all the box and are quite easy
to remove. In other models, they are
only about 50mm long and are quite
tricky to remove. To make matters
worse, there is nothing in the manual
covering this procedure.
The secret lies under a small plastic
label, marked “AC IN”. This can be
peeled off and it is then possible to
see how the cover and body are slotted
together. In all models, the construction is quite compact and this makes
them a little diffi­cult to work on but
not seriously so.
As I have already noted, the mains
fuse (F1101) was intact and there
were no other visible signs of damage.
Well, it didn’t take long to establish
that bridge rectifier D1102 was open
circuit; a rather strange fault in the circumstances – I would have expected
a short circuit.
Anyway, that’s how it was. And,
since I didn’t carry this item in stock,
it would have to be ordered. But what
else could have been damaged? The
next major component after the bridge
rectifier is a small IC (IC1101), which
is described as an auto voltage selector
– more about this later.
There was no conclusive way to test
this IC without a new rectifier but I
made a few resistance measurements
across the appropriate pins and, judging by the transistor configurations
shown within the IC, I suspected that
it may be faulty also. So that was added
to the order list.
So what about the NV-J1A machine?
Like the NV-FS90A, it would almost
certainly need some replacement
parts. Closer exami­
nation revealed
one important difference between the
FS90A and the J1A; the J1A was fitted
with a protective thermistor (D1101)
across the mains, a refinement the
FS90A lacked, in spite of its up-market
price tag. I hoped that this might have
prevented further damage.
And yes, this had broken down
and taken out the fuse. So maybe
that was all that was needed. I had
a replacement on hand, fitted it, and
tried again. No joy; another fuse for
the garbage bin.
I moved on to the bridge rectifier
and confirmed that this was the real
culprit – it had broken down. So another bridge was added to the order.
Next to consider was the auto voltage
selec­tor IC, which was a different type
from that in the FS90A. Be­cause the
J1A had apparently suffered a heavier
wallop than the FS90A, I also ordered
a replacement for this IC, just in case.
So the order was duly despatched
and a couple of days later I had the
spares on the bench. Naturally, I went
straight to the FS90A and fitted the
new rectifier. I made a quick check
but no joy. Well, I’d suspected the auto
voltage IC anyway and it was easy to
fit the new one, so that was the next
step. In fact, when I pulled the old
one out and compared measurements
with the new one, my suspicions were
confirmed.
Unfortunately, when the new one
was fitted there was still no response,
so it was back to basics. Measuring
August 1994 41
So that was something I had learned
– the hard way – and is well worth
jotting down for future reference. It
could save an embarrassing bounce.
Back to the J1A
across the bridge rectifier showed
300V plus, which seemed logical, and
so I moved on to pins 3 and 4 of the
auto voltage selector IC, which should
have had essentially the same voltage
between them. In fact, it was zero.
From there it was no great effort to
find the culprit. R1125, a 2.2Ω safety
resistor in the negative line from the
rectifier, was open circuit. Well, that
was no great problem; 2.2Ω safety
resistors are a readily available component and I always have stock on hand.
A new one was fitted and the
machine came to life. A thor­
ough
workout confirmed that all functions
were performing cor­rectly and I gave
the machine the usual clean and lube
routine before ringing the owner with
the good news.
A nasty bounce
He came in the following morning,
which was a Saturday, and collected it.
And I naturally assumed that that was
end of that story. Not so – he was on
the phone first thing Monday morning
with the bad news. The thing was dead
at first switch-on.
It wasn’t a good way to start the
week. All I could do was ask him to
bring it back in and assure him that I
would sort it out.
Back on the bench, it didn’t take
long to find the reason; the 2.2Ω safety
resistor I had fitted had failed. I could
find no obvious reason for this, so I fitted another one, switched on, and the
machine came good. I put it through
several on/off cycles – the previous
failure had obviously occurred at
switch-on – and it came good every
time.
42 Silicon Chip
I left it for a while to attend to other
jobs, then tried again. Bingo! It was
completely dead. And yes, it was the
2.2Ω resistor. But why?
With no obvious clues I realised I
needed help. I could spend a week
trying to puzzle this out and more
than likely be no further advanced
at the end of that time. I rang one of
my con­tacts at Panasonic and put the
problem to him.
His reaction was immediate. “What
kind of safety resistor are you using?”
I replied that it was a standard 2.2Ω
1W type such as one buys at the (electronic) lolly shop; the kind of thing
everyone uses.
“But not from Panasonic?”
“No – does that matter?”
“You fit a Panasonic type and you
wont have any more trou­ble.”
Sensing a certain amount of incredulity on my part, he went on to
spell out the difference (more on that
in a moment). In any case, I had little
option; I ordered the type he nominated – several, in fact – and when they
came to hand fitted one to the FS90A.
And that really was the end of it; the
machine has given no further trouble.
The crucial difference
So what is so magical about the
Panasonic component? It is a wire
wound type, as distinct from the more
usual metallised types. And, although
it is rated at only 1W, as are the other
types, it is capable of withstanding a
much heavier switch-on surge. And
this particular power supply does
have a heavy switch-on surge. This, in
turn, is a byproduct of the auto voltage
selector system.
In the meantime, I had gone back
to the J1A. I had already replaced the
thermistor, established that the bridge
rectifier had failed, and suspected that
the auto voltage selector IC might be
faulty. In fact, this latter suspicion
proved to be correct.
So the rectifier and IC were replaced
and, on a hunch, I checked the 2.2Ω
safety resistor. I wasn’t really surprised
to find it had failed and was grateful
that I had the correct replacement type
on hand.
And with all that attended to, the
machine came to life. More importantly, it operated in all modes – an
important point, because lightning
strikes don’t always stop at the power
supply. They can pick on odd components anywhere in the system and create faults which can be very difficult
to track down.
So the owners of both the FS90A
and the J1A were lucky; the damage
in either case could have been much
more serious.
Strike three
Machine number three was an NVFS65A. It had also suffered a lightning strike and, as it transpired, had
suffered more damage that the other
two. And, of course, with the first two
under my belt, I was feeling pretty
cocky about this one, the power supply
being essentially the same.
But pride goes before a fall. I went
through the same rou­tine: the fuse,
the rectifier, the safety resistor and the
IC, all of which needed replacement.
But it still wouldn’t deliver voltage
and I began digging deeper into the
circuit.
I didn’t have much luck initially.
Panasonic suggested some likely
components to be either tested or
replaced, as did a colleague who was
also familiar with these units. But none
of the suggestions helped.
So I was at a temporary dead end.
I say temporary, because I was quite
confident that I could track down the
trouble, given time.
But the owner had other ideas. He
called in to see how things were going
and I told him quite frankly that the
job had proved far more difficult than
thermistor across the mains and the
surge protector at the power point. Yet
it was still damaged.
On the other hand, nothing else
in the house was damaged, nor was
there any indication that any of the
surge protectors had been activated.
And by “activated” I mean destroyed
because, as far as I know, all these
surge protectors are sacrificial devic­
es; once activated they have to be
replaced.
Fig.2: the power supply for the
Panasonic NV-J1A video recorder is
basically similar to that in the FS90A
but also features a surge protection
thermistor (D1101).
I had originally anticipated.
“Well,” he said. “What about fitting
a new power supply?”
This was the last thing I would
have suggested. I told him that yes,
I could do that, but it would prove
pretty expensive.
He shrugged his shoulders. “Hang
the expense. If that will solve the
problem, go ahead and fit it”.
So that’s what was done. In fact,
Panasonic didn’t supply a complete
replacement; just a new board to fit
in the metal case. And that put the
machine back in operation.
But while having the machine fixed
was gratifying in one sense, it was
somewhat frustrating in another. I was
still keen to know what the problem
really was and anticipated that I could
probably retain the old board and solve
the puzzle at my leisure.
But that was not to be either. Before
I could even raise the matter the owner
indicated that he wanted to take it and,
since it was his property, there was
nothing I could do about it. So that
one must remain a mystery.
Surge protectors
Still on the subject of lightning
strikes, I mentioned earlier that the
owner of the J1A was a keen computer
buff. The significance of this is that he
had become more than usually aware
of the risk of power line surges, of
whatever origin, to his precious computer equipment.
As a result, he had fitted surge protectors to most of his power outlets,
including the one normally used for
the video recorder. So the recorder
had two levels of protection; its own
Machine number 4
Machine number four was a model
NV-L20 and it came in shortly after
the first three. It was completely dead
also but there was no suggestion of a
lightning strike. On the other hand, the
failure did look as though it might be
linked to a mains shut-down, at least
indirectly.
As the owner explained, the machine had been operating normally
immediately prior to a planned maintenance shut-down by the supply authorities. All householders had been
warned and there was no great hassle
involved.
However, when power was restored
a couple of hours later, the machine
was dead. I didn’t attach much importance to this initially, writing it
off as mere coincidence. And it might
have been too but what I found made
me think.
With the previous jobs still fresh in
my mind I went through the power
supply in short order; the fuse, safety
resis­tor, and rectifier were all intact,
with the usual 300V plus out of the
rectifier. Nor could I find anything
obviously wrong with the voltage
selector IC.
With all those items cleared, suspicion fell on the starter network.
This consists of four resistors (R1102,
R1103, R1123, and R1124) across
the rectifier output and a 1µF 400V
electroly­tic capacitor from the junction of R1103 and R1123. The arrange­
ment is broadly similar to many starter
circuits used in TV sets. And a common fault is an open circuit or high
value resistor.
In this case, however, all the resistors checked out OK, leaving the
capacitor as the prime suspect. And so
it proved to be. When I pulled it out
and checked it, I could get no reading
at all on the capacitance meter.
So that was an easy one; a new capacitor and the machine came good.
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August 1994 43
SERVICEMAN’S LOG – CTD
I gave it a routine clean and lube and
returned it to the customer.
But where did the mains shut-down
come into this? I have no doubt that
the capacitor was on its last legs anyway and it is important to appreciate
that power is applied to this part of
the circuit at all times, whether the
machine is being used or not. Only
when power is turned of at the mains
is the voltage across the capacitor
removed.
My theory is that this voltage
contributed to a certain amount of
“forming” of the capacitor – enough
to maintain a small amount of capacitance which was sufficient to allow the
system to start. Removing this voltage
for a couple of hours was the last straw
that sank the camel’s hump. But, of
course, it’s only a theory.
Auto voltage selector
Finally, this might be as good a time
as any to expand on the auto voltage
selector system. It is designed to allow
the machine to work on a very wide
range of voltages, although there is no
mention of this in the manual. The
specifications simply say “230-240V,
AC, 50-60Hz.”
However, a practical test confirmed
that it can be plugged into 110V (which
I have available in the workshop) and
still operate quite normally, without
any adjustments.
There is nothing especially new
about this concept; it has been around
in many TV sets for several years now,
although the particular circuit configuration was new to me. Also, it is not
something we think much about in
this country, being blessed with a 240V
50Hz standard which is used virtually
everywhere through­out the continent.
But from a manufacturing and marketing point of view, the advantages
are obvious. Not only does the one
power supply suit all countries but it
is even useful within some countries.
This would be particularly so in countries like Japan, which has a variety
of power supply systems in different
areas. While the 110V 60Hz system is
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44 Silicon Chip
the one most commonly used, many
areas use 220V 50Hz.
I also understand that there are some
220V 60Hz and 110V 50Hz systems.
It all adds up to a real nightmare, not
only for manufacturers and distributors but also for consumers who wish
to move from one area to another. And
to further complicate matters, the same
type of power outlets are often used
for both voltage systems. It doesn’t
take much imagination to visualise
SC
the problems this can cause.